Pressure exchangers



pt. 15, 1959 R. D. PEARSON 7 2,904,243

PRESSURE EXCHANGERS Filed June 2a, 1956 United States Patent 2,904,243PRESSURE EXCHANGERS Ronald D. Pearson, Chesterfield, England ApplicationJune 28, 1956, Serial No. 594,462 4 Claims. (Cl. 230-69) This inventionrelates to machines, hereinafter referred to as pressure exchangers, inwhich each of a plurality of cells serves cyclically to receive gas froma source of lower pressure and discharge it to a pressure-increasingmeans, and to receive gas from said pressure-increasing means anddischarge it to a region of lower pressure. The cells are arrangedaround the periphery of a rotor mounted to pass over appropriatepermanently-open ports in the walls of a stator. (Of course, the termsrotor and stator are used relatively, the one to the other, so that itmight be that the rotor is stationary in space and that the statorrotates about the rotor.) The admission and discharge of the gas to andfrom the cell in the lower and in the higher pressure stages ishereinafter referred to as scavenging; being defined as a condition inwhich both ports of the cell being open together fora sufficientduration of time, there occurs a displacement of a substantial part ofthe former contents from the cell, and their replacement by fresh gas.

The pressure-increasing means is conveniently a combustion chamberwherein the received gas is made to burn with a fuel to increase bothits volume and temperature.

Conveniently, too, but not necessarily, the motion of the gas into andout of the cell in both of the scavenging stages is unidirectional, sothat it is possible to speak of an inlet to and an outlet from the cell,the inlet being on one flank of the rotor and the outlet on the otherflank.

When the machine is arranged as an engine, it serves to convert some ofthe pressure energy from said pressure increasing means into kineticenergy.

It is desirable that immediately before a cell reaches the low pressurestage or the high pressure stage, the gas in in the cell should beaccelerated towards that port, usually the cell outlet, from which thegas is to be discharged in said stage. Hereinafter this acceleration isreferred to as prescavenging.

A somewhat fuller exposition of the working of such machines may befound in my copending patent application Serial No. 594,461.

It will be understood that at the instant when one of the ports of thecell is opened, either to a region of higher pressure than thatobtaining in the cell or to a region of lower pressure than thatobtaining within, then a wave will travel through the cell from thatnewly-opened port towards the other port, at a velocity comparable withthat of the velocity of sound, being in the first case a compressionwave and in the second case a rarefactio'n wave. When the wave reachesthe far end, it will be reifiected: if the far end is closed, thereflected wave will be of the same sense as the incident wave,compressioncompression or rarefaction-rarefaction: while if the far endis open, the reflected wave will be of opposite sense. In the same waywaves are generated at the instant of closing of a port which had beenopen.

It is one of the objects of the present invention to provide for theneutralisation of waves before their existence can operate adverselyupon the performance of the machine.

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The following description relates to the accompanying drawing whichshows, by way of example only, one em bodiment of the invention.

In the drawing is shown a development of therotor: and stator system ofa pressure exchanger embodying the The rotor cells Each of the similarcells C in the rotor R has an inlet port Ci and an outlet port C0, thelatter'having its-ad! jacent cellwalls bent backwards.

The stator ducts As the cell C moves from right to left its inlet portCi sweeps successively over the mouths of the following inlet ordelivery ducts, namely: low-pressure scavenging Li,

high-pressure scavenging Hi, and low-pressure prescavenging PLi.Similarly the outlet port C0 sweeps over the mouths of a' number ofducts, all but one being outlet or receiving ducts. They are: the lowpressure scavenging outlet or receiving duct Lo, the high pressurecompression delivery duct CH1, the high pressure pre-scavengingreceiving duct PHo, the high pressure scavenging receiving duct Ho, andthe low pressure prescavenging receiving duct PLo.

The pressure-exchange cycle Thus the cycle through which such acell Cpasses con-. sists of five stages, of low and high pressure scavengingand pro-scavenging and of compression. More particue larly, this cycleis as follows:

(1) Low pressure scavenging: When the cell is at the righthand side ofthe drawing, its inlet and outlet ports Ci and Co are both open to therespective low pressure scavenging delivery and receiving ducts Li andL0. This is the low pressure scavenging stage; the outlet L0 dischargesto the atmosphere and the linlet Li receives air from the atmosphereeither directly or via a fan.

(2) Compression: From the low-pressure scavenging stage L the cellpasses to the compression stage where the cell inlet Ci is closed whilethe outlet Co is open to the delivery duct CHi.

(3) High pressure pro-scavenging: In the high-pressure pro-scavengingstage PH, the cell inlet Ci is open to the high pressure delivery ductHi and the cell outlet Co is open to the receiving duct PHo. j

(4) High pressure scavenging: High pressure scavenging exists when bothcell inlet and outlet are connected to the respective high pressureducts Hi and Ho. The receiving duct Ho leads to a combustion chamber(not shown) and the delivery duct Hi takes from the combustion chamber.

(5') Low pressure pie-scavenging: Beyond the high pressure scavengingstage is the low pressure pre-scaveng ing stage PL. Here the cell inletCi is at first closed and is then opened to the inlet PLi. The outlet isopen firstly to the prescavenging outlet PLo, and then to the lowpressure scavenging outlet Lo.

Wave propagation At the instant of opening or closing either of the cellports, a wave is set up in the cell from that port, and travels throughthe cell, at a speed comparable with that of sound, towards the othercell port, where it' is re flected. Such waves are either compressionwaves or rarefaction waves according to the change occurring at theoriginating end, and are reflected as either compression waves orrarefaction waves according to the condition obtaining at the reflectingend.

The path of a compression wave in space, i.e. relative to the stator, isshown by a pair of continuous lines and that of a rarefaction wave by apair of broken lines; the first line of each pair marks the foot of thewave and the second line marks the head. As is known, compression wavestend to grow steeper as they progress, and rarefaction waves to growshallower, so that the two full lines of a pair converge, while brokenlines diverge. A compression wave is transformed into a shock wave whena pair of full lines meet.

The occurrence of a wave may be advantageous or deleterious; and thepresent invention is concerned with the neutralisation of waves that aredeleterious.

A certain wave pattern will exist for one set of conditions only, thecondition exerting the most influence on the wave pattern being therotor speed. In the drawing, therefore, the wave pattern shown is thatobtaining when conditions approximate to design values; however, it isso arranged and it is one of the objects of this invention that thechange in pattern caused by variation in operating conditions has only aminor effect upon the performance of the machine.

The wave cycle The following are the waves that are developed in thecell in its cyclic movement from low pressure scavenging throughcompression, high pressure prescavenging and scavenging and low pressureprescavenging back to low pressure scavenging.

In the drawing, as a cell C moving from right to left approaches the endof the low pressure scavenging stage L the flow of gas through the cellis retarded by waves of compression 3 and 4 travelling from the celloutlet Co towards the cell inlet Ci and generated by the gradual closingof the cell outlet C to the low pressure scavenging discharge duct L0,i.e. the wave 3 by a partial and the wave 4 by the complete closure. Thereception of wave 3 at the inlet end Ci causes substantial cut-off ofthe flow into the cell from the stator inlet duct Li, but without thatreversal of flow into duct Li which would occur if no partial closure ofL0 preceded the complete closure, so that waves 3 and 4 coincided andtheir combined amplitude therefore were greater.

The waves 3 and 4 are desirable waves, since by them the energyremaining in the cells after scavenging is substantially used to producea supercharging effect; but the reflected waves 6 and 7 form ararefaction pulse which if further reflected would be undesirable. Theyare substantially neutralised on reception at the pre-compressionnozzles CHi feeding the outlet ends of each cell.

Precompression is effected by introducing gas from duct CHi at anelevated pressure so that compression wave is produced. This wave isreflected from the closed inlet Ci as wave 8 producing furthercompression. Although Waves 5 and 8 are desirable, the furtherreflection of wave 8 from the outlet end would be undesirable. Bysuitable design of the nozzles in precompression duct CHi to givesufficient pressure drop the desired neutralisation of wave 8 iseffected. The pressure of gas fed to the nozzles is then equal to thatof the wave head of wave 8.

After transit of wave 8 the cell contains stagnant gas at an elevatedpressure substantially equal to that in prescavenge receiving duct PHo,so that the cell contents are not influenced when the cell is opened tothis duct on further movement of the rotor.

' Prescavenging is produced by opening the cell inlet Ci to the highpressure scavenging delivery duct Hi, when a compression wave 9completes the compression process and accelerates the cell contents toscavenging speed. The prescavenging receiving'duct PHo is so positionedas to receive wave 9 and on arrival this Wave causes di charge from thecell to commence. By restriction of the cell outlets preferably by usingbent back trailing edges as at C0 the reflection of wave 9 as ararefaction wave can be at least reduced.

On passing the wall beyond PHo, the cell enters the high pressurescavenging stage H and to prevent the formation of a pressure pulse bytemporary flow stoppage the width of the wall is made less than the cellwidth or less than the width of subsidiary channels if fitted in theoutlet end of the cell.

Hot gas enters the cell at its inlet end from duct Hi and cold gas iswithdrawn from the outlet end H0; the progress of the interface betweenthe two gases is indicated line 2.

At the end of the high pressure scavenging stage the scavenging flow isarrested in two stages by rarefaction waves 10 and 12. The first wave 10is produced by partial restriction of the cell inlet caused by thechange to a high angle inlet nozzle of the delivery duct Hi. Wave 12 isproduced by complete cut off from delivery duct Hi. Termination ofscavenging in two stages as described is desirable in order to improvethe flow condit-ions at the receiving duct H0, especially at lowerspeeds of operation. Also, the high angle nozzle serves to neutraliseany unwanted residual waves received by it.

An important rarefaction wave 11 is produced upon opening the celloutlet Co to duct PLo at a lower pressure and is supplemented byreflection of wave 10 from the wall between H0 and PLo. In the same waythe final rarefaction wave 15 produced by opening the cell to the lowpressure scavenging discharge duct L0 is reinforced by the reflection ofwave 12.

Wave 11 reflects from the closed inlet end as wave 13 and is followedclosely by compression wave 14 produced by opening the cell inlet to thelow pressure prescavenging delivery duct PLi. An undesirable retardingpulse 13, 14 is thereby produced which will be continually reflectedfrom the cell ends during scavenging as indicated for the first suchreflection by pulse 16, 17, and which increases in width as the speed isreduced. Substantial neutralisation of wave 15 by impingement on thenozzles PLi is ensured by correct design of the nozzles PLi, gas for PLiis fed from expansion duct PLo.

The dividing wall between PLi and Li is of narrower width than the cellin order to prevent production of an undesirable rarefaction pulse.

The interface between incoming fresh gas from inlet Li and spent gas isshown progressing along a cell by line 1.

The apparatus and its operations having thus been described, it remainsto point out how that apparatus embodies the features of the presentinvention.

Thus it will be seen that at a number of stages of the cycle, a waveWithin a cell is substantially neutralised on reaching a cell port bymeeting a gas stream which is of appropriate pressure relative to thepressure obtaining in the cell and which comes from a duct ofappropriate throat area to which said port is then open. In particular,there occurs a substantial neutralisation of the rarefaction wave 15when it reaches the inlet port Ci of the cell at a moment when that portis open to the low-pressure prescavenging delivery duct PLi, said ductPLi having a substantial angle of inclination to the normal to thedirection of movement of the cell inlet Ci, and that inclination beingin the direction of movement of the cell port Ci.

Again the pressure wave 3 generated at the cell outlet C0 by flowrestriction and reflected as rarefaction wave 6 at the open cell inletCi is substantially neutralised on again reaching the cell outlet Co,and the pressure wave 4 generated at the cell outlet C0 by the closingthereof to the low pressure receiving duct Lo and reflected from theclosed cell inlet Ci as pressure wave 7 is substantially neutralised onagain reaching the cell outlet C0 both neutralisations being due to themeeting of the respective waves at the cell outlet Co with a gas streamof appropriatev pressure relative to. the pressure obtaining in the cellfrom the duct PHi of appropriate throat area to which the cell outlet Cis then open.

The compression wave 9 generated at the cell inlet Ci by the openingthereof to the high pressure delivery duct Hi is partially neutralisedon reaching the cell outlet C0 by reason of the cell outlet C0 being ofrestricted throat area as compared with the cell being then open to thereceiving duct PHo through which gas is taken to the delivery duct PHito which the cell outlet C0 is open when the cell inlet Ci is closedafter leaving low pressure scavenging Li.

. The compression wave generated at the cell outlet C0 by the openingthereof to the higher pressure delivery duct PHi between the lowpressure and high pressure scavenging stages L0 and H0 and reflected ascompression wave 8 from the closed cell inlet Ci is substantiallyneutralised on again reaching the cell outlet C0 because outlet Co isstill open to said duct PI-Ii and by the correct choice of nozzle throatarea CHi the pressure developed behind wave head 8 is made substantiallyequal to that at inlet to the nozzles. In other words, the cell outletis open to the said duct PHi for a sufiicient duration to allow of thewave 5 to be reflected as Wave 8 into said duct PHi even at the highestspeed of rotation.

The wave-neutralising pocket PH between the low pressure and highpressure scavenging stages is so arranged that the receiving duct PHothereof is open to the cell outlet C0 when the pressure wave 9originating with the opening of the cell inlet Ci to the high pressuredelivery duct Hi arrives at the outlet C0, while the delivery duct CHiof said pocket is open to the cell outlet C0 before the receiving ductIHo. The pocket PH is shown Wholly closed external to the machine butoperation is characterised by the same description when a substantialbleed from this pocket for some external use is-allowed.

It will of course be understood that the invention may take many otherforms than that shown in the accompanying drawing. In particularalternative arrangements giving more eflicient working are obtained whendelivery duct CHi faces the rotor periphery or the cell inlets Ci.

With peripheral admission a pressure wave will travel radially inwardsto be reflected at the closed wall formed by the rotor drum and besubstantially neutralised after travelling radially outward to meet thenozzles CHi, neutralisation being achieved in a manner similar to thatdescribed with reference to Fig. l.

In order to allow for the effect of linear taper of a cell as measuredin the direction of wave travel, a configuration inherent in the use ofperipheral admission, it is found convenient to use the same equationsdefining the required nozzle as for cells of constant section in thedirection of wave travel the effective cell cross sectional area fortapered cells being taken at a section one quarter of the distance fromthe said nozzles to the opposite closed wall.

Also the rarefaction pulse formed by Waves 6 and 7 remaining from lowpressure scavenging is partially neutralised by the gas stream flowingfrom nozzles PHi although in this case the flow is normal to thedirection of motion ofthe said pulse.

With nozzles CHi facing the cell inlets Ci neutralisation of pressurewaves is as described with reference to Fig. 1 except thatneutralisation takes place at the cell inlet Ci instead of cell outletC0 after reflection from the closed cell outlets C0. Also therarefaction pulse formed by waves 6 and '7 is substantially neutralisedby nozzles CHi after its further reflection from the cell outlet Co.

A disadvantage of the expansion stage as shown in Fig. 1 exists wherebyby the reflection of rarefaction wave 11 as wave 13 and the subsequentformation of pressure Wave 14 an unwanted rarefaction pulse is formedwhich becomes increasingly conspicuous as speeds are reduced. This canbe minimised by extending the high pressure inlet duct Hi to a pointclose to prescavenge duct PLi the said duct extension taking the form ofa highly inclined nozzle wall.

Further unwanted pulses can in certain cases be prevented from formingby correct choice of duct dividing wall thickness. In particular wallsbetween outlet ducts PI-Io and Ho and between PLi and Li are best havingwidths less than the cell wall spacing such that undesired momentaryflow reversal associated with transfer of a cell port between ducts atdifferent pressure is prevented, whilst at the same time the productionof unwanted pressure or rarefaction pulses which would occur whenmomentarily closing such a cell port is avoided. The Width of such wallsis required to be from 0.2 to .8 of the cell spacing for a wall betweenPHo and H0 and from zero to .8 of the cell spacing for a Wall betweenPLi and Li depending on flow velocities.

The invention is not limited to the use of a single precompressiondelivery duct CHi and further advantages can be obtained when a numberof delivery ducts are employed. In particular the addition is desirableof at least one delivery duct opening to the cells after lowpressurescavenging but before opening to a main delivery duct of much largerextent. The said first delivery duct is arranged to have a pressurehigher than that in the cell but lower than that in any succeeding ductbetween low and high pressure scavenging whereby a pressure wave iscreated without creating the excessive initial speeds of gases deliveredfrom the duct nozzles which would occur if the main delivery duct wereopened directly to cells leaving low pressure scavenging. The firstducts are so arranged that together with the main delivery duct a singlepressure wave of steadily increasing amplitude is formed to be reflectedat the opposite closed wall and received again by the said main deliveryduct neutralisation of the reflected pressure wave being exactly asdescribed with reference to Fig. 1.

The nozzle angles required to give neutralisation of waves in accordancewith the foregoing description are substantially as given by thefollowing equations:

(1) Compression stage CHi: Nozzle angle B required to produce a pressurebehind Wave 8 equal to that upstream of the nozzles.

Cell cross sectional area measured normal to axial direction at, a pointone quarter of the Aw distance from nozzle to opposite wall n Nozzleoutlet cross sectional area measured normal to axial direction Axial isreplaced by radial when duct nozzles are open to the cell periphery.

B=nozzle angle measured from axial direction.

a=cell helix angle measured from axial direction at point of measurementof Aw except for duct nozzles facing the cell periphery when =zero.

- =ratio of specific heats for gas at constant pressure and constantvolume.

an=speed of sound of gas issuing from nozzles.

aw=speed of sound of gas i tially in cells when at the same pressure asthat issuing from nozzles.

(2) Low pressure prescavenging nozzles PLi: Nozzleaiigle-B required toproduce neutralisation of a rarefaction wave of amplitude Z Z when thegas initially in the cells at Z has the same pressure as that upstreamof the'noz'zles at Z,,, i.e.

Using the same symbols but any value for Z as defined by the wave pointto be neutralised.

What I claim is:

1. In a pressure exchanger, a rotor having a plurality of cells arrangedperipherally thereof and opening on opposite sides of the rotor, andwalls on opposite sides of said rotor each having at least one ductcommunicating with the openings of the cells, the duct in the firstwallbeingsupplied with gas for delivery to the cells and the duct in thesecond wall receiving gas from said cells, the ducts in the two oppositewalls being in operative alignment, whereby the cells in their passagefrom one duct to the other will he suddenly closed at least at one endto set up a Wave in the cell which is then reflected from the oppositeend, and means for neutralizing said reflection comprising a furtherduct communicating with the 'one end of the cell and a supply of gasthereto at a pressure suflicient to neutralize said reflection andprevent further reflection from said one end.

2. A pressure exchanger comprising a rotor having a plurality ofradially extending partitions about the periphery thereof to form aplurality of cells which are open at opposite sides of the rotor, astator having a wall adjacent each side of the rotor, each wall havingducts therein communicating with the openings in the cells, the ducts ina first wall defining delivery ducts and those in the secondwalldefining receiving ducts and being substantially in operativealignment with each other, means for supplying scavenging gas at a lowpressure to a first delivery duct, means for supplying scavenging gas ata high pressure to a second delivery duct,- a low pressurepre-scavenging receiving duct in the second wall between high and lowpressure scavenging receiving ducts in the second Wall and substantiallyopposite a wall portion between the high and low pressure scavengingdelivery ducts, the wall portion between the low pressure pre-scavengingreceiving duct and the high pressure scavenging receiving duct facingsaid cells and being .2 to .8 of the width between cell walls, such thatno undesirable pressure pulse is formed by momentary flow from the highpressure receiving duct to a cell faced by the wall portion, wherebyreflections are prevented.

3. A pressure exchanger comprising a rotor having wall adjacent eachside of the rotor, each wall having. ducts therein communicating withthe openings in thecells, the ducts in a first wall defining deliveryducts and those in the second wall defining receiving ducts and beingsubstantially in operative alignment with each other, means forsupplying scavenging gas at a low pressure to a first delivery duct,means for supplying scavenging gas at a high pressure to a seconddehvery duct, a low pressure pre-scavenging duct in the first'wallbetween the high and low pressure scavenging delivery ducts, saidpr'e-scavenging duct being smaller than and in'alignment'with a'portio'nof the low'pressure scavenging receiving duct and in communication withthe cells, the wall portion between the low pressure pre-scavenging ductand the low pressure scavenging delivery duct facing said' cells andbeing .1 to .8 of the width between cell walls such that no rarefactionpulse is produced within the cell faced by the wall portion due tomomentary flow from the cell to the low pressure delivery duct, wherebyreflections are prevented.

4. A pressure exchanger comprising a rotor having a' plurality ofradially extending partitions about'the periphery thereof toform aplurality of cells which are open at opposite sidesof the rotor, astatorhaving a wall adjacent each side of the rotor, each wall having ductstherein communicating with the openings in the cells, the ducts in afirst wall defining delivery ducts and those in the second wall'defining receiving ducts and being substant'ially in operative alignmentwith each other, means for supplying scavenging gas at a low pressure toa first delivery duct, means for supplying scavenging gas at a highpressure to a second delivery duct and a further duct in the second wallbetween the high and low pressure scavenging receiving ducts fordelivering gas at high pressure to the cells, a high pressurepre-scavenging receiving duct in the second wall in communication withthe cells, positioned between the further duct and the high pressurescavenging receiving duct and in communication with the further duct,the Wall portion between the prescavenging receiving duct and the highpressure scavenging receiving duct facing said cells and being .2 to .8of the width between cell walls, such that no undesirable pressure pulseis formed by momentary flow from the high pressure receiving duct to acell faced by the wall portion, whereby reflections are prevented, andmeans to neutralize reflections due to closing a cell as it approachesthe high pressure second mentioned delivery duct, comprising means forsupplying gases at such a pressure to said high pressure prescavengingdelivery duct as to neutralize' such reflection and prevent furtherreflection from Ethe cell which is open to the high pressureprescavenging uct.

References Cited in the file of this patent UNITED STATES PATENTS

